At present, a most widely applied coaxial cable access solution is an access networking solution based on a Data over Cable System Interface Specification (DOCSIS) technology. The DOCSIS technology mainly supports transmission of a data packet between a computer network and a cable television network, and between a cable television head-end and a user. An access data transmission system based on the DOCSIS technology is shown in FIG. 1, and includes a cable modem termination system (CMTS) device 101 and a cable modem (CM) device 102. The CMTS is an optical line terminal providing broadband access, and the CM is a terminal device. The CMTS is connected to the CM using a hybrid fiber coaxial (HFC) network in a tree form, and the CM is connected to a user. The access data transmission system provides data backhaul and downloads channels respectively using an uplink band and a downlink band of the cable television network.
In a DOCSIS standard, it is specified that a downlink data transmission channel corresponding to CMTS multicast supports 16 profiles, a downlink data transmission channel of the CM supports 4+1 profiles (four private profiles and one general profile), and an uplink data transmission channel of the CM also supports a plurality of profiles. The downlink channel is modulated using orthogonal frequency division multiplexing (OFDM), and a subcarrier is adaptively modulated. Usually, an OFDM channel whose bandwidth is 192 MHz may be divided into 4096 subcarriers or 8192 subcarriers, and carrier spacings are 50 KHz or 25 KHz respectively. Adaptively modulating the subcarrier refers to determining, according to channel quality of a corresponding adaptive channel, of a quantity of bits carried on a subcarrier. For example, channel quality at a location of number 12 subcarrier may be relatively good, and a modulation scheme of 4096 QAM is used to carry 14-bit data. However, because a channel at a location of number 3000 subcarrier is most greatly interfered, only 1024 QAM modulation can be used to carry 12-bit data. In this way, quantities of bits carried on different subcarriers are different. Therefore, multicast profiles supported by a CM using a subcarrier to transmit data are different. In this way, how to select a multicast profile is an important technology of concern.